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Title: Shear deformation of highly viscous magmas : a textural study of strain localisation
Author: Shields, Jessica
ISNI:       0000 0004 5920 4630
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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Whether silicic volcanoes erupt explosively or effusively is controlled by the ability of magma to lose gas prior to eruption. However the mechanisms behind permeability development and obsidian generation remain an enigma for volcanologists. Torsion experiments were performed at high temperature and pressure on three-phase rhyolitic magma analogues simulating magma deforming in shallow conduit conditions. Significant loss of porosity and dissolved water occurred due to brittle fracturing of the melt. Outgassing efficiency increases with strain and is enhanced by crystal content. Fabric development as a result of strain localisation impacts sample rheology as much as crystal, bubble or water contents. Experiments suggest multiple fractures must have formed and healed during shear to account for observed water loss. Viscous suspensions of particles and bubbles were sheared to observe, in real time, the development of deformation fabrics. Bubble deformation and crystal rearrangement are enhanced by increasing crystal content and strain-rate, which magnify heterogeneities in distributions of particles and the gas phase. Development of bands of high particle content and low strain adjacent to melt-rich, high strain areas illustrate the first stage in the formation of permeable outgassing networks and reveals the governing role of textures on magma rheology and outgassing. Multiple methods were used to study outgassing and emplacement mechanisms of the Rocche Rosse obsidian flow. Strain localisation and highly localised variations in melt water content are responsible for extreme textural heterogeneity observed on micron to m scale in the flow. The formation of repeated flow bands may result from high sensitivity of obsidian degassing to water content. Complex surface deformation as opposed to conduit processes governs water and textural distribution and prolongs flow. A model is proposed consisting of multiple cycles of shear-induced outgassing, densification and re-vesiculation during ascent and emplacement to result in observed textures and water contents.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available